I have no idea how to fix this as I don't even know where the leak is coming from. It is very wet in the sill and I have to use a towel to soak it up. Rubbish at DIY. Don't have lots of money, but would pay some one to fix this.

If it's a T25 & leaking around windows in rear side or rear of van then suspect rotted window edge where rubber sits. You will need to remove window which is easy & see what state it is in. If metal has rotted away you have 2 choices. Have it welded ie do job proberly or bodge it if you can with PU sealant.